118 Dr. J. Erskine-Murray. 



more of itself on the plate. A liquid is quite unsuitable, since every 

 particle of the metal probably retains particles of the liquid adhering 

 to it. Thus the smell of a liquid remains long after the plate appears 

 to be quite dry ; and it has been found that such a film as must exist 

 to cause the odour is quite sufficient to alter the potential very 

 considerably. 



12. In order to obtain uniformity of action the clean glass-paper 

 or emery-cloth used to polish a plate was fixed on a wooden roller 

 made to revolve with circumferential velocity of about 100 cm. 

 per second. Care was taken to hold the plate so that the scratches, 

 caused by the polisher, should all be parallel. Thus little or no grit 

 could lodge in the surface, which would have occurred had the 

 scratches crossed one another. A record was kept of the nature of 

 the polishing agent in every case. A piece of glass-paper, or other 

 polisher, was seldom used more than once or twice, and was never 

 used for any different metal. 



13. M. Pellat found that every change in the smoothness of a 

 surface is accompanied by a change in its contact potential ; but as 

 his experiments were limited to metals washed with alcohol, it was 

 of interest to extend them to more general cases. Thus a plate of 

 zinc which had been polished on clean glass-paper, and had therefore 

 a surface sharply scratched in parallel lines, was found to be 



0'70 volt 



positive to the standard gold plate. It was next burnished with a 

 tool of hardened steel, and with the same standard plate it now gave 



0-94 volt, 

 Two hours later the same plate gave 



0-92 volt, 



showing that the effect is nearly permanent. By burnishing the 

 plate again a finer polish was obtained. With the same standard the 

 potential is now 



1-00 volt, 



still further burnishing giving 



1-02 volt. 



If the zinc be now polished on glass-paper it returns to its original 

 potential. Now steel, of which the burnisher was made, is negative 

 to zinc ; hence this rise cannot have been due to particles of steel on 

 the zinc, but must have been caused by some change involved in the 

 smoothing process to which the zinc had been subjected, possibly by 

 a hardening of the surface layer. 



